My N900 did not have this problem before the update, but it appeared during the update.

As the update downloaded and installed, I checked on it every couple of minutes. Sometime in between checks, the phone became completely unresponsive. I couldn't tell if it was actually off or not. Plugging it into the charger did not activate the notification led or cause the device to boot. The power button did not have any effect whether it was plugged in or not. After reading this thread, I realized that it had probably shutdown but been unable to restart. Pulling the battery solved the problem: the phone booted and completed installing the update.
I then shut down a second time to see if the problem was persistent. It wasn't- now the phone powers up normally.
So my phone briefly had symptoms identical to your own. I have no idea if the cause of my problem is similar to your own.